1. Field of the Invention
This device relates to devices for measuring force and particularly to a device that measures the force exerted by the flexure spring that supports a magnetic recording head.
2. Prior Art and Informatio Disclosure Statement
Disk files for magnetic recording include a one or more magnetic recording heads held against a rotating memory disk. The surface of the head facing the moving disk surface is contoured so that an airbearing is generated at the interface (the region between the two surfaces) that maintains the separation of the two surfaces. The separation for present day disk files is less than five microinches. It must be maintained very accurately, i.e. within a microinch in order to maintain a uniform predictable recording signal from the head and in order to help prevent contact between the two surfaces that would lead to damage of the recording surface. The force that maintains the head at this spacing from the disk is very small, typically grms. and must be maintained at a precise value.
The standard construction for supporting the head is a leaf spring or flexure of a thin metal sheet, typically about an inch or so long and one half an inch wide in todays diskfiles. The head is secured at one end of the broad side of the flexure facing the disk. The other end of the flexure is clamped to the arm of an actuator that positions the head on the desired recording track on the disk surface. The precise value of force applied by the head against the disk can only be achieved by accurately positioning the clamping location with respect to the disk surface and by accurately controlling various factors that influence fexing of the flexure. These factors include, dimensions of the flexure (including thickness), the location of the head on the flexure relative to the clamping location, residual stresses in the flexure that might affect flatness of the flexure in the unstressed condition, etc. Because of the criticality of the head force load and the numerous factors that must be controlled to achieve the required head load force, it is standard practice to test the flexing properties of each head-flexure assembly before it is built into the diskfile assembly.
A device and method for testing the force characteristics of the head flexure assembly has been developed by the Automatech Corporation located in Santa Cruz, Calif.
The test device includes a reference plate having a plate reference surface.
A load cell is mounted on the plate reference surface and has a cell reference surface against which the force to be measured is applied thereby generating an electronic signal. The signal is displayed as force against the cell surface expressed in grams.
The device is calibrated by resting a precisely known calibrating weight on the cell reference surface then adjusting electronic parameters of the measuring circuit until the displayed value of force equals the calibrating weight. This procedure ensures that the cell display will present an accurate measurement of the force exerted against the cell reference surface.
Next, a test yoke is placed on the reference surface straddling the cell reference surface so that a reference surface on the yoke faces the cell reference surface. The legs of the yoke have been precisely machined so that the difference between the height of the reference surface on the yoke above the plate reference surface and the height of a flexure clamp surface above the plate reference surface equals a value corresponding to the distance of the flexure clamping surface on the file from the disk surface. Then a screw connecting the cell to the reference plate is turned to bring the cell reference surface into contact with the yoke reference surface. Contact of the cell reference surface with the yoke reference surface is that position of the screw where the display of force jumps from zero to a value greater than ten milligrams (since the display of present devices will read no smaller than ten milligrams and the present screw arrangement allows for no controlled adjustment finer than ten milligrams.
The steps described in the two preceding paragraphs are the steps required to calibrate the force testing device to display the correct force when the flexure is clamped in the test device with the head in contact with the cell reference surface so that the flexure is bent an amount equal to the bend that is imposed on the same flexure when mounted in the disk file.
The quality control procedure is to place each head-flexure assembly in the calibrated test fixture so that the test device will display a force that must be within prescribed limits in order for it to be accepted for mounting into a disk file. The device according to the prior art displays a value of force out to hundredths of a gram thereby requiring a requirement to calibrate the device to within a hundredth of a gram.
The present trend in disk files is toward ever smaller disk files requiring smaller head loading forces and calibration procedures that are mor precise than ten milligrams.